The present invention relates in general to communication systems, and is particularly directed to a modification of the framing pattern employed in the transport of integrated services digital network (ISDN) communications over a two-wire telephone wireline channel, to replace selected portions of the framing structure in order to provide for the transport of a secondary or auxiliary plain old telephone system (POTS) channel.
Although ISDN is a digital subscriber loop (DSL) technology that provides for the integration of both voice and data onto a single subscriber loop, ISDN subscribers often maintain a separate analog (POTS) line for voice communications. For example, a xe2x80x98work at homexe2x80x99 telco customer may have an ISDN line for business purposes, and employ a separate analog line for private residential use. This dual line configuration may present an installation problem to the telephone service provider, as it requires the provider to provide two subscriber loops which may be problematic due to the limited number of subscriber pairs that may be available. It would be desirable to have the flexibility of supplying both POTS and ISDN services over a single subscriber loop.
Pursuant to the invention, this objective is successfully achieved by usurping selected portions of the framing pattern that make up a repeated data framing mechanism employed in the transport of integrated services digital network (ISDN) communications over a single two-wire telephone wireline channel, and replacing the these usurped portions of the framing pattern with data bits of a compressed secondary POTS channel for transport over that same two-wire pair.
Currently defined industry standard (ANSI directive T1.601-1992) framing structure for 2B1Q ISDN signaling operating at a data rate of 160 kilobaud (kb), specify that 128 kbps are available for customer (bearer (B1, B2) channel) data, and 16 kbps are available for control or data (D) channel. Of the remaining 16 kbps of non-payload bandwidth (e.g., eoc, M4, CRC, etc), 4 kbps are used for overhead maintenance data. The remaining 12 kbps of the non payload bandwidth are used to transport a synchronization framing pattern, made up of an eighteen bit sync word repeatedly transmitted every one and a half milliseconds and used to define basic frame boundaries. A superframe contains eight basic frames and is defined by inverting the first or initial sync word for the first basic frame of the superframe.
The invention takes advantage of the fact that recently developed ISDN communication methodologies are capable of operating at very low bit error rates, which allows replacing a portion of the 12 KHz framing bandwidth with an auxiliary (compressed) voice channel, without suffering an unacceptable increase in synchronization errors. For this purpose, the conventional framing structure is modified to include only the (36) sync word framing bits of selected (e.g., the first and fifth) basic frames of a respective superframe. The framing bits of the remaining six basic frames are replaced an 8 kbps compressed secondary or auxiliary POTS channel which, as a result of recent advances in voice compression techniques, is sufficient to transport toll quality voice.
For an 8 kbps channel, sixteen of the available eighteen bits of each of the six usurped basic frame sync words are replaced with two bytes of the compressed auxiliary voice channel. The remaining two bits of each usurped basic frame sync word are set to a prescribed value (e.g., xe2x80x981xe2x80x99), so as to leave only one kbps of unused data bandwidth. To implement the invention, each end of an ISDN communication system, such as one providing extended range service between a central office site and a customer premises set is augmented to include an associated auxiliary channel interleaving transceiver unit, which controllably replaces the usurped basic frame data with the 8 kbps compressed voice channel.
This framing data-usurping transceiver unit comprises a transceiver (such as a Total Reach transceiver manufactured by Adtran Corp., Huntsville, Ala. (Total Reach being a Registered Trademark of Adtran Corp.). This transceiver is coupled to interface standard 2B1Q ISDN communication signals with the local ISDN-dedicated loop, and to synchronously interface 2B+D ISDN data with a framer/deframer unit. Synchronous serialized data transfers of eighteen bits of (144 kbps) 2B+D customer data are effected in accordance with a clock signal that is locked to a network 8 KHz clock.
The framer/deframer unit has a transmit port coupled to a digital/analog interface and is operative to output 144 kbps of scrambled 2B+D user data, 4 kbps of maintenance, 3 kbps of framing, 1 kbps of unused data, and an (8 kbps) auxiliary compressed voice channel (CV), that is provided by way of a voice compression-decompression circuit chip. The voice compression-decompression circuit is operative to compress a 64 kbps uncompressed voice channel (provided by an associated xcexc-law codec and SLIC that are interfaced with an auxiliary POTS channel) into an 8 kbps compressed voice channel. To accommodate a continuous 8 KHz clock used by the voice compression chip to transfer data, an N-bit by 1-bit FIFO is coupled in the transmit and receive data paths of the framer.
On the loop transmit side, the framer/deframer unit includes a transmit slip buffer that is employed to ensure synchronization with network timing of the customer and the compressed auxiliary voice channel data, which tend to be xe2x80x98burstyxe2x80x99. The transmit slip buffer is coupled to the transmit link of the bidirectional port of the transceiver and receives frame sync control from the transceiver""s supervisory microcontroller. The slip buffer is also coupled to supply 2B+D data and modified framing pattern data to a transmit multiplexer. The transmit multiplexer is coupled to receive maintenance data from the transceiver control bus and 8 kbps compressed voice channel data from the transmit/receive FIFO. The multiplexer interleaves each of its data inputs into an outgoing data stream which is scrambled into a scrambled serial data stream for application to the loop.
On the loop receive side, the framer/deframer unit has a receive slip/elastic buffer coupled to receive 2B+D data and modified framing data supplied from a demultiplexer (deframer) unit, to which descrambled serial data stream received from the loop is applied. The demultiplexer also outputs maintenance data and couples compressed voice data supplied through the FIFO to the voice compression-decompression circuit. As an incoming scrambled data steam from the far end of the loop is descrambled by data descrambler, it is demultiplexed into a 144 Mbps customer ISDN (2B+D) channel, an 8 KHz auxiliary composed POTS data, 4 kbps maintenance channel and a 3 kbps modified framing data channel. The remaining unused 1 kbps channel associated with the two xe2x80x981xe2x80x99 bits of each usurped basic frame is used only for timing synchronization. The 144 kbps ISDN customer data is output as a standard 144 kbps 2B1Q ISDN channel by the U transceiver over the local ISDN tip/ring loop. The auxiliary POTS voice channel decompressed from 8 kbps to 64 kbps and coupled to a local tip/ring POTS interface via the xcexc-law codec and associated SLIC.